WO 2009/067365 A3 discloses a method in which nanofibers, by means of electrospinning, are deposited wet or moist onto a substrate layer. In this context, the nanofibers fuse with each other at the crossing points. The exact process control is however very complex and is difficult to realize.
Moreover, an electroblowing method is known in which nanofibers are blown under pressure. Compared to the generally known electrospinning methods, only fiber layers with high variance in regard to the fiber diameters of the individual fibers can however be realized by means of the electroblowing method. Thus, two principally different production processes are concerned.
Moreover, EP 1 940 531 B1 discloses to solidify nanofibers of low melting point by means of thermal calendering and to bond them with a substrate layer. However, in this context, the nanofibers fuse with each other and with the fibers of the substrate layer practically at all crossing points.
The most commonly used variant for bonding a nanofiber layer with a substrate layer is realized by use of a bonding agent. Depending on the application method of the bonding agent, often the formation of sail-like projections, forming an undesirable air resistance, between individual fibers on the filter surface is to be expected in this context. Due to different surface properties, generally a matching bonding agent system must be developed for different substrate layers. Moreover, the employed bonding agent must be resistant relative to the medium to be purified; otherwise, it can dissolve, for example, in case of liquid filtration, and the nanofiber layer can become damaged as a result.